This invention relates to mold fluxes and their use in the continuous casting of steel.
In the continuous casting of steel a mold flux is generally added to the surface of the molten steel in the mold. The flux provides lubrication between the mold wall and the steel, it reduces the loss of heat from the surface of the steel, it protects the surface from oxidation, and it may remove impurities such as alumina from the steel.
As granules evolve much less dust compared with powder, mold fluxes used in the continuous casting of steel are often used in the form of granules, which may be produced by, for example, spray-drying of the flux constituents. The excellent flowability of granules makes them particularly suitable for automatic feeding to the mold, for example, using a DAPSOL.RTM. feeder. However once the flux is in the mold the flowability of the granules becomes a disadvantage since the granules tend to find their own level under high rates of flow of steel into the mold and the surface of the steel may become exposed in the corners of the mold.
It has now been found that the above problem can be alleviated if the granules contain a minor amount of an expandable material which will expand under the action of heat and will cause the granules to break down into powder on the surface of the steel. According to the invention it has also been found that spherical granules yield the best results, and that the expandable material (particularly acid treated graphite) should have a particular size, and utilize particular binders, in order to obtain the best results. Also, it has been recognized according to the present invention that in continuous casting of ultra low carbon (ULC) steel that the combination to be utilized should be different than for other types of steel, the insulating properties of the mold fluxes being especially critical for ULC grade fluxes, and carbon pickup must be minimized, and that according to the invention spherical granules can be used for ULC steels even though the conventional wisdom is that granules do not insulate as well as powders and, therefore, are not suitable for use with ULC steels.
The basic granular mold flux of the invention comprising refractory metal oxide, one or more fluxing agents, a binder and an expanding agent, the expanding agent being present in an amount of 0.1% to 3% by weight based on the weight of the flux, preferably about 0.3 to 1%, and the granules are in spherical form. Spherical granules have the best properties in terms of chemical uniformity and cold flowability and also have suitable insulating ability. However, conventional spherical granules in the past have not been as forgiving in the mold as powders during turbulent conditions. During turbulent conditions the narrow face is particularly disturbed by rolling and level variation and spherical granules tend to run down toward the lower levels due to their good flowability. This can result in exposing liquid flux or even steel near the narrow face. However, because of the expanding agent according to the invention, as well as the reduced average particle size of the spheres, the permeability of the flux is reduced, thereby improving its insulating values, and the cold flowability is reduced, the net result being that the material can be used successfully during submerged entry shroud (SEN) and tundish changes without the tendency to form steel floaters.
According to one aspect of the present invention a mold flux is provided comprising refractory metal oxide, at least one fluxing agent, a binder, and expandable graphite, said expandable graphite having a size of less than about 80 mesh, and said flux in the form of spherical granules. The granules preferably have a size of 200-500 microns, which is a smaller range than for conventional spherical granules, and the expandable graphite comprises 0.3-1.0% by weight of the mold flux.
It is also highly desirable according to the invention to provide a soluble carbonate as a binder, preferably either sodium carbonate (soda ash) or lithium carbonate. At least 4% soda ash, or at least 2% lithium carbonate, or a combination of at least 2% soda ash and at least 1% lithium carbonate, are typically used. Most desirably the binder comprises between about 8-14% by weight soda ash, or between about 4-7% by weight lithium carbonate, or a combination of soda ash and lithium carbonate wherein double the percentage of lithium carbonate plus the percentage of soda ash is between about 8-14% by weight.
According to another aspect of the present invention a mold flux is provided containing the basic constituents as set forth above but also including starch in a sufficient amount so as to cause carbon black to migrate to the surface of the granules to improve efficiency of carbon black addition, reducing slag rim, improving thermal insulation, and reducing carbon pickup; and MnO.sub.2 (oxidizing agent) in sufficient amount so as to oxidize carbon and reduce carbon pickup allowing higher carbon addition to the flux providing improved thermal insulation and less slag rim. The amount of starch is about 0.1 to 1.0% by weight, for example about 0.3 to 0.7% by weight, typically about 0.5% by weight, and the amount of MnO.sub.2 is about 1 to 5% by weight, for example about 2 to 4% by weight, typically about 3% by weight.
According to a further feature of the invention there is provided a method of continuously casting molten steel in a mold the method comprising adding to the mold prior to, during or after teeming of the molten steel a spherical granular mold flux comprising at least one refractory metal oxide, at least one fluxing agent, a binder and an expanding agent, the expanding agent being present in an amount of 0.1% to 3%, preferably 0.3 to 1%, by weight based on the weight of the flux. That is, according to the invention a method of continuous casting molten ultra low carbon steel is provided using a casting mold, the method comprising the step of adding to the mold prior to, during, or after teeming of molten ultra low carbon steel a spherical granule mold flux comprising refractory metal oxide, at least one fluxing agent, a binder, and expandable graphite, starch in a sufficient amount so as to cause carbon black to migrate to the surface of the granules to improve efficiency of carbon black addition, reducing slag rim, improving thermal insulation, and reducing carbon pickup; and MnO.sub.2 in sufficient amount so as to oxidize carbon and reduce carbon pickup allowing higher carbon addition to the flux providing improved thermal insulation and less slag rim.
It is the primary object of the present invention to provide for the continuous casting of molten steel utilizing a fluxing agent that has most of the advantages recognized for granular fluxes in the prior art, with fewer of the drawbacks, and is particularly well suited for continuous casting processes, including for ULC steel. This and other objects of the invention will become clear from a inspection of the detailed description of the invention and from the appended claims.